In the past, nitrile group-containing copolymer rubber (acrylonitrile-butadiene copolymer rubber) has been used as a material for rubber parts for automobile use such as hoses, belts, and tubes by making use of its oil resistance, mechanical properties, chemical resistance, etc. Further, nitrile group-containing highly saturated copolymer rubber obtained by saturation by hydrogenation etc. of carbon-carbon double bands in the polymer main chain of nitrile group-containing copolymer rubber is further excellent in heat resistance and is being used for rubber parts such as seals, belts, hoses, and diaphragms.
In view of such a situation, Patent Document 1 discloses a nitrile group-containing copolymer rubber composition containing a nitrile group-containing copolymer rubber obtained by hydrogenating carbon-carbon unsaturated bonds of an unsaturated nitrile-conjugated diene copolymer and a vulcanizer, where the nitrile group-containing copolymer rubber has an iodine value of 80 or less, has a Mooney viscosity (ML1+4, 100° C.) of 30 to 100, and contains 1 to 20 wt % of gel. According to the art of this Patent Document 1, the rubber composition is excellent in processability, and a cross-linked rubber excellent in original physical properties such as tensile stress and compression set resistance is obtained.
On the other hand, in recent years, the demands for quality in the market has been rising. For example, the loads in various applications such as seals, belts, and rolls have become higher. From the viewpoints of larger tensile stress, smaller compression set, and reduced deterioration due to heat buildup and energy loss, a cross-linked rubber excellent in low heat buildup property has come to be sought. However, the cross-linked rubber obtained by cross-linking the nitrile rubber composition described in Patent Document 1 is not necessarily sufficient in low heat buildup property. To meet the recent trend toward higher loads, further improvement of the low heat buildup property has been sought.